Single crystal silicon, the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (“Cz”) method. Using the Cz method, crystal growth is commonly carried out in a crystal-pulling furnace, wherein polycrystalline silicon (“polysilicon”) is charged to a crucible and melted by a heater surrounding the crucible. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by extraction via a crystal puller. Crucibles used in conventional crystal pullers are commonly constructed of quartz because of its purity, temperature stability and chemical resistance. Natural quartz, even high quality, contains various metallic impurities and is not entirely satisfactory in respect of purity. If metallic impurities contaminate high-purity single silicon crystals, the performance of the semiconductors produced therefrom is adversely affected. Thus, high-purity synthetic quartz glass powder has been used for making crucibles.
One method for making high-purity synthetic quartz glass powder is via the sol-gel method using alkoxysilane as a raw material. In this process, however, 100% of the alkoxy groups are not reacted and some of them remain in the dry gel as bound alkoxy groups. Also, the alcohols generated as by-product of the reaction are partly left in the dry gel, for a carbon concentration in the dry gel of up to 1,000 ppm. When such carbon contaminant intermixes in a synthetic quartz powder product, it may give rise to CO or CO2 gas causing foaming or bubbles, when the powder product is fused for molding into a crucible or an ingot. The bubble-containing quartz crucibles may cause various problems, such as deterioration of dimensional stability in use at a high temperature and bursting of the bubbles during growth of single crystals causing crystal defects.
Applicants have developed a process for controlling/improving bubble stability in quartz crucibles by controlling the amount of impurities in the synthetic silica feed. A number of methods have been proposed to reduce impurities by treating synthetic quartz powder at a high temperature. U.S. Pat. No. 5,516,350 discloses a method to reduce the carbon concentration by treating the synthetic quartz powder in an atmosphere with an oxygen concentration of >30 vol. % at a temperature of at least 1000° C. JP Publication No. 06-040713 discloses treating synthetic quartz glass powder with gaseous chlorine or a chlorine-containing gas at an amount of 0.1 to 10% Cl at a temperature of 1000 to 1500° C. to reduce the OH level to ˜45 ppm. U.S. Patent Publication No. 20030124044 discloses treating synthetic silica powder in a helium atmosphere of 700 to 1400° C. for at least 1 hour, to reduce the carbon residual to less than 2 ppm and the OH group to less than 50 ppm.
There is still a need for an economical and efficient method to reduce bubble density and improve bubble stability in crucibles by controlling impurity levels in synthetic silica powder feed. Furthermore, there is still a need to reduce impurity levels in synthetic silica powder feed for use in the manufacture of optical and semiconductor components such as crucibles, glass